It is known in the art to make ferromagnetic cores for electric motors by stacking a plurality of metal laminations. In particular, the cylindrical cores of the stators and rotors of these motors are made by blanking substantially ring-shaped laminations from a sheet of ferromagnetic material and then packing the laminations in a suitable number for obtaining a core with the desired axial length.
In particular, especially with regards to stator cores, the prior art also includes making the core in the form of a plurality of segments that are arranged according to a rectilinear sequence configuration to facilitate the winding of the coils around the respective field poles. Once the endings are completed, it must be possible to bend the rectilinear configuration to assume a closed-circle shape so that it can be inserted inside the shell or cylindrical casing of the electric motor.
For example, patent application EP-A1-0871282 describes a stator core wherein the rectilinear sequence of segments is obtained by blanking the laminations to simultaneously reproduce all of the shapes corresponding to the sections of the segments of the rectilinear sequence, and by keeping the shapes united along a deformable peripheral portion. In other words, the lamination stack leaving the blanking mold has the form of a rectilinear sequence of segments, in which each segment is connected to the adjacent segment by a thin deformable membrane.
Even though a suitable configuration is obtained to partially facilitate the subsequent winding of the coils, this solution has various drawbacks. First of all, because blanking work is along the linear development of the sequence of field poles, this solution limits the possibility of obtaining stator cores beyond a certain diameter.
Even if a blanking mold able to process sheets of large width was made, there is the risk of obtaining a not very satisfactory or totally unusable final result, mainly due to the differences in thickness that will inevitably be encountered in the sheets between the two side edges.
In addition, it should be borne in mind that the connection between the segments created with a thin membrane of unsheared material can be subject to breakage during the subsequent steps of coil winding, bending the core from the rectilinear configuration to the circular one and/or during the step of inserting the complete stator into the casing or shell of the motor. The winding of the coils could in fact require a fold between the segments in the opposite direction to that subsequently contemplated for taking the sequence from the rectilinear configuration to the circular configuration. Bending in different directions could thus cause the thin membranes that connect each segment to an adjacent segment to break.
Patent application EP-A1-0833427 describes another example of embodiment of a ferromagnetic core composed of a plurality of separate segments that can be mechanically connected to each other. Each lamination stack that constitutes a segment is made separately from the other segments of the same core. The laminations comprise at least one protruding engagement portion and at least one concave engagement portion having mutually complementary shapes to allow engagement with the respective engagement portions of adjacent laminations.
This document indicates how to work on a narrower sheet with respect to that known from the previous document. This also allows the various segments of a core to be assembled in a rectilinear configuration that can then be bent to give a circular configuration.
However, none of the various embodiments presented in this document proposes solutions suitable for avoiding relative axial sliding between adjacent segments. Furthermore, several of the proposed embodiments envisage the mechanical deformation of particularly thin engagement portions, which are difficult to produce with the necessary precision and are particularly delicate during the course of the subsequent steps of mechanical coupling between the segments, winding the coils and bending the core into the circular configuration.
In theory, this solution should also allow the creation of a single, continuous winding, namely by using a continuous coated or enameled conductor that extends without interruption to form all of the coils of the field poles. Even if this could give sufficient cohesion to all the segments of the stator core during bending from the rectilinear configuration, in which the single continuous winding of all coils is carried out, to the closed circular configuration suitable for being housed in the shell or casing of a motor, any break in the protruding engagement portions of the segments subjected to deformation would result in discarding the entire stator and, in consequence, an undesired increase in production costs.
Besides the solutions proposed by application EP-A1-0833427, this document underlines a particularly important aspect in the manufacture of ferromagnetic cores made in the form of segments mechanically coupled to each other. In fact, in order to reduce the magnetic reluctance of the core as much as possible, clearance between the various segments, and in particular between the respective protruding and concave engagement portions, must be reduced to the minimum. However, this requires high machining precision and also high fitting force on the engagement portions of the segments when they are coupled together.